Q&A: Professor discusses new approaches to the implementation of the quantum internet

Researchers around the world are working on a network that can connect quantum computers with each other over long distances. Andreas Reiserer, Professor of Quantum Networks at the Technical University of Munich (TUM), explains the challenges that need to be mastered and how atoms trapped in crystals can help.

Professor Reiserer, what is the quantum internet and how does it differ from the classical internet as we know it?

The idea is the same: We use today’s internet to connect computers to each other, while the quantum internet allows quantum computers to communicate with each other. But in technical terms, the quantum internet is much more complex. This is why only smaller networks have been realized yet.

Why do we need a quantum network?

There are two main applications: First, networking of quantum computers makes it possible to increase their computing power; second, a quantum network will make absolutely tamper-proof encryption of communications possible. But there are also other applications, for example network telescopes to achieve a previously impossible resolution to look into the depths of the universe, or the possibility of synchronizing atomic clocks around the world extremely precisely, which makes it possible to investigate completely new physical questions.

How do quantum computers exchange information?

For the most part, exactly the same way as in the classic internet: Using photons. These photons are transmitted through optical cables. In the classic internet, very strong signals are used, pulses of light consisting of billions of photons. Here, the information is transmitted using a binary code: Light on or light off, similar to Morse code.

However, the quantum internet is different: It still uses a binary code, but the information is not carried by pulses of light with many photons, but rather by individual photons. This makes it possible to transmit quantum mechanical states that carry extremely large amounts of information.

Why is it so much harder to build a quantum internet?

Photons are lost on their way through the optical cable. In a normal network, signals can be easily amplified using repeaters that add more photons to the light pulses. But in the quantum internet if a single photon goes astray, all information transmitted is irretrievably destroyed. This type of loss is the biggest problem when building a working network. It could be solved using quantum repeaters, which my group is working on right now.

What challenges are you facing?

Transmission over short distances already works very well. But the loss grows exponentially as distances increase. To build quantum repeaters, we divide the total distance into many small sub-segments. Buffers, actually tiny quantum computers, store the quantum state after each subsegment until a photon has been sent to the next subsegment.

Then what is called quantum teleportation can be used to then “forward” the information to the transmitted photon. To do this requires efficient small quantum computers, which we are developing.

What do these tiny quantum computers look like?

The best systems investigated still use individual atoms that are trapped in a vacuum with laser light and cooled to very low temperatures. However, this approach requires an entire laboratory full of optical components, making it difficult to implement this approach on a small scale.

Instead, we use silicon crystals where the individual atoms have been inserted and you can say are trapped in the crystal. The erbium atoms we use retain their quantum mechanical properties under these conditions. This structure also requires low temperatures, but it is technically much, much simpler.

We have been able to show that this system works in principle and that the erbium atoms, when excited, generate photons suitable for transporting quantum information. A major advantage here is that we can build thousands or even millions of these structures on a single silicon chip.

Why is it important?

The need for buffering in the repeaters would mean that it would take a very long time to transport information from one location to another. To achieve a faster speed, we use what is called multiplexing. This means that the process is carried out as many times as possible in parallel. Our technology makes this possible and we are already working on the realization.

Will we all use the quantum internet in the future?

The situation may turn out to be similar to the classic Internet: At first, hardly anyone could imagine that today everyone would walk around with an Internet connection in their pocket, use satellites to determine our location and navigate with the help of the Internet. We are still in the very early stages of the quantum internet.

Our current research is still about the basics, looking at things like: Can we connect these systems? Can we succeed in spreading quantum states all over the world? The potential of this type of system as we know it today would already be revolutionary for some areas, and I’m sure there will be a great many applications that no one even thinks about today.